Protection against electrical shock

ABSTRACT

To reduce the danger of electrical shock in two-wire nongrounded electrical systems, the leakage current from line L1 to ground is balanced by an injected current from ground to line L1, and similarly the leakage current from line L2 to ground is balanced by an injected current from ground to line L2. In this way, the leakage current from line L1 to ground cannot pass through a human body from ground to L2, since the leakage current form a closed current loop.

United States Patent I151 3,670,206 Sircom 1 June 13, 1972 [54] PROTECTION AGAINST ELECTRICAL 3,319,123 5/1967 Scanlan ..317/18 D SHOCK 3,320,480 5/1967 Failor ...3l7/l8 D 3,515,941 6/1970 Moore et al ....'..3l7/l8 D [72] Inventor: Richard Cumming Sircom, Windsor, Nova A 9 Canada Primary Examiner James D. Trammell [73] Assignee: Eastech Limited Attorney-Stevens, Davis, Miller & Mosher [22] Filed: April 12, 1971 ABSTRACT 5 [211 App] No 8 To reduce the danger of electrical shock in two-wire nongrounded electrical systems, the leakage current from line Ll pp D118 to ground is balanced by an injected current from ground to 0m. 6, 1970 Canada ..o94,ss2 line and Similarly the 8= current from line L2 to ground is balanced by an injected current from ground to line 52 us. Cl ..317/1a D, 317/27 R, 317/33 R 5 I [5 l Int. Cl. ..H02h 3/28 In this wa A y, the leakage current from line L1 to ground cannot [58] Field oiSeaI-ch ..317/l8 R, 18D, 272435;, p through a human body from ground to L2 since the leakage current form a closed current loop. 5 21 18 Drawing Figures UNITED STATES PATENTS 2,700,125 1/1955 King et a1 ..3l7 /l8 D s! ue "-1 ili SCI{ tu I 02 "1 T)- I IO C 7 I m: A lu L I I /II 0-!!! ,l m in :3 la n o!!! PATENTEBJun 13 I972 SHEET 10F 8 FIG. M8

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CURRENT Mk? SUPPLY P3 PROTECTION AGAINST ELECTRICAL SHOCK The present invention is an improvement of the invention disclosed in applicants early U.S. Pat. application No. 060,887. i

The present invention relates to an improvement in the invention described and claimed in our copending Canadian Pat. application No. 060,887, filed Sept. 2, 1969 and U.S. Pat. application No. 833,463, filed June 16, 1969.

That earlier invention relates to protective means adapted to reduce the danger of electrical shock in two-wire nongrounded electrical systems.

Systems of this general type are commonly used in situations where the danger caused by an electrical shock is greater than would normally be the case.'Thus in wet conditions, such as in association with swimming pools, where a very good ground connection can exist from the body of a human, and in conditions such as during surgical operations where electrical apparatus is used on particularly sensitive parts of the human body, a current can flow through part of a human body which is sufficient to cause death.

Existing systems are effective to reduce considerably the danger of the passage of relatively large ground currents, but it has been found that often a relatively small ground current can also be lethal. The dangerous level of current is so low that even the relatively small capacitive line-to-ground leakage current in 'a normal system is large enough to be fatal.

An object of the said earlier invention is the provision of protective means adapted to reduce the danger of electrical shock in two-wire non-grounded electrical systems, and capable of providing protection even against the relatively small capacitive line-to-ground leakage currents.

According to the earlier invention, in protective means adapted to reduce the danger of electrical shock in a two-wire non-grounded electrical system, current generating means cause a first current to be injected between ground and a first of two lines of the system, this current being substantially equal to and in phase with a first leakage current between that first line and ground, and the current generating means cause a second current to be injected between ground and the second of the two lines of the system, this current being substantially equal to and in phase with a second leakage current between that second line and ground, whereby the part of the first leakage current which can flow from ground through a body to the second line, and the part of the second leakage current which can flow from ground through a body to the first line, are substantially reduced.

The object'of the present invention is to provide an improved protective means which operates basically as set out in the previous paragraph, but which involves the use of less equipment,

According to the present invention, in protective means adapted to reduce the danger of electrical shock in a two-wire non-grounded electrical system, current generating means cause a first current to be injected between ground and a first terminal, this current being substantially equal to and in phase with a first leakage current between a first of the two lines of the system and ground, and in which the current generating means cause a second current to be injected between ground and a second terminal, this current being substantially equal to and in phase with a second leakage current between the second of the two lines of the said system and ground, and the said terminals being connected to at least one of the two said lines, whereby the part of the first leakage current which can flow from ground through a body to a second line, and the part of the second leakage current which can flow from ground through a body to the first line, are substantially reduced.

The invention will now be described, by way of example, with reference to the accompany drawings, of which FIGS. I through 6B are copied from said earlier application, and in which:

I FIG. 1 is a diagrammatic representation of a two-wire nongrounded electrical power supply system, and of protective apparatus applied thereto;

FIGS. 2 to 5 are explanatory diagrams relating to leakage currents and the present invention;

. power system through a contact breaker panel FIGS. 6A and 6B, when arranged side-by-side as indicated in FIG. 6C, together show the circuit diagram of a capacitiveleakage-current suppressor shown in FIG. 1;

FIGS. 7 through 10 are explanatory circuit diagrams relating to a modification of the circuit shown in FIG. 5;

.FIGS. 11 through l3 are circuit diagrams showing respectively three ways of putting the embodiment of FIG. 10 into practice; and

FIGS. 14A and 148, when arranged as in FIG. 14C, form a circuit diagram of a complete system utilizing the arrangement of FIG. 13. I

Referring first to FIG. 1, an isolating transformer 1 having a primary winding P1 connected across an alternating current supply 3 and having a grounded electrostatic screen 5, has a secondary winding S1 which is used to energize an isolated The contact breaker panel 6 is orthodox in that it includes two insulated bus bars 7 and 9 connected respectively to the two ends of the secondary winding S1, and a plurality of automatic overload contact breakers CB1, CB2, and CB3. A main manual circuit breaker MB is arranged to permit breaking of both connections to the bus bars 7 and 9, and each circuit breaker CB1, CH2, and CB3 also is effective to break both poles of the supply.

The contact breakers CB1, CB2, and CB3 are arranged to control the supply of alternating current electrical power to separate sub-circuits SCI, SC2 and SC3 and typically each sub-circuit would supply a number of two-pin or three-pin power outlets 10 into which portable or movable electrical equipment could be plugged as and when required.

Systems such as have been described so far are well known in the art and are used in environments, such as hospitals. where it is desired to eliminate the risk of ground fault currents. Since both poles of the supply are'isolated from ground, no large ground current can flow unless two faults to ground exist, one from each pole of the isolated supply. However, such systems are inherently defective in that there is a capacitive coupling between each live part of the system and ground, and although the capacitive currents which flow per foot run may be small, they total to a considerable current in a large system.

In the system shown in FIG. 1, Le, the system of the earlier invention, contact breaker CB1 is used solely as a power supply for a capacitive-leakage-current suppressor 11 and a ground leakage current indicator 13. The suppressor 11 includes a first set of input terminals Al, A2, and A3 for connection respectively to a first pole of each of the subcircuits it is desired to monitor, and a second set of input terminals B1, B2 and B3 for connection respectively to the second poleof those subcircuits. The connections to terminals Al and B1 are actually internal leads of the suppressor 11. As indicated in FIG. 1, the suppressor 11 includes a series of variable potentiometers' RlAl, RlA2, RlA3, R181, R182 and R183 and these are associated respectively with the input terminals Al to B3. Associated with the group of terminals connected to first pole of the supply, i.e., the terminals A1, A2 and A3, is a variable potentiometer R24A and associated with the group of terminals connected to the other pole of the supply is a variable potentiometer R24B.

FIG. 2 is a diagram showing how a capacitive leakage current can flow through leakage capacitance CX and a resistive leakage current can flow through leakage resistance RX from line L1 to ground and thence through any available impedance Z to the other line L2. FIG. 3 illustrates how the similar leakage capacitance CY between line 12 and ground together usually form the impedance Z. The direction of current flow is, in view of the altematingpotential involved, also alternating. Referring back to FIG. 2, it will be seen that if a human body l-IB bridges the insulation between line L2 and ground, it shunts the impedance Z, and the leakage current between line L1 and ground now passes partly through that human body HB. If the impedance of the body is low compared with impedance Z, which will often be the case, the leakage current will flow for the most part through the human body I-IB.

and leakage resistance'RY I Referring now to FIG. 4, it will be seen that a first human body 1181 connected between L2 and ground would shunt the impedance 22 presented by leakage capacitance C2 and leakage resistance R2, and would pass the leakage current from line L1 to ground on to line L2. On the other hand, a second human body HB2 connected between L1 and ground would shunt the impedance 21 presented by leakage capacitance C1 and leakage resistance R1, and would pass the leakage current from line L2 to ground on to line L1. The operation of the suppressor 11 is indicated schematically in FIG. 5. Ideally, the suppressor produces a current 11 from ground to line L1 which is exactly equal to leakage current 11 from'line L1 to ground. Since there is no net current flow from line L1 to ground, a human body l-IBl connected between ground and line L2 will pass no current. Similarly, ideally the suppressor produces a current 12 from ground to line L2 which is exactly equal to leakage current 12 from line L2 to ground, a human body I-IB2 connected between ground and line L1 will pass no current. It will be understood that the presence of body I-IBl must not occur at the same time as the presence of bodyI-IBZ, since then both bodies would carry currents. I

The suppressor 11 (see FIGS. 6A and 68) includes a power supply 51 and two sub-circuits 53 and 55 which are associated respectively with line L1 and line L2. The power. supply 51 provides separate d.c. supplies at +200 v., v., and 200 v. to sub-circuit 53 and to sub-circuit 55. It also supplies a common two Figures, leads A to G on each Figure are connected to the .corresponding leads on the other Figure. The components used in suppressor 11 include the following:

Transistors Resistors OIAI RlAl 10,000ohms. Q2 2N37ll RlA2 10,000 ohms Q3 RIAJ 10,000 ohms 04 R1131 10,000 ohms Q5 2N3702 RIBZ 10,000 ohms Q6 2N5415 R1133 10.000 ohms Q7 2N3440 Q8 2NS415 RZAl 4,700 ohms Q9 2N5415 Q10 2N3440 R3A1 100,000 ohms Q11 2N3440 R3A2 100,000 ohms R3A3 100,000 ohms Diodes R4 10 megohms D1 1N5212 R5 100,000 Ohms D2 1N5212 R6 82,000 ohms D3 1N52l2 R7 8,200 ohms D4 1N5212 R8- 4,700 ohms R9 15,000 ohms CR1 1N914 R10 4,700 ohms CR2. 1N914 R11 470 ohms CR3 1N914 R12 470 ohms CR4 VRIBO R13 4.700 ohms CR5 VRIBO R14 470,000 ohms CR6 3842 R15 470,000 ohms CR7 384Z R16 470,000 ohms .CR8 3842 R17 470,000 ohms (R9 3842 R18 4.700 ohms R19 330,000 ohms Capacitors R20 4,700 ohms R21 220 ohms Cl-0.0033 MFD R22 220 ohms C2 250 MFD R23 100 ohms C3 0.1 MFD R24A 2 megohms C4 20 MFD R248 2 megohms C5 20 MFD C6 50 MFD C7 50 MFD- Considering first sub-circuits 53, terminals A1, A2 and A3 are indicated, and it will be noted that a lead 57 connects the tiometer RlAl, the line L1 is connected to ground when switch SlAl is closed. SlAl is one set of contacts of a multiple switch, other sets of contacts being indicated at S1A2,

input line L1 to the terminal A1 while a lead 59 connects the input line L2 to terminal B1. Thus the suppressor uses one of its inputs to monitor the power supply to the suppressor.

Through the series combination of capacitor C1 and poten- SlA3, SlBl, SlBZ and SlB3. Transistor Q1A1 has its base connected to the slider of potentiometer RlAl, and its emitter is at very close to ground potential, since its base is connected to ground through resistor RlAl. This emitter is connected to a differential amplifier formed by transistors Q2 and Q3, and the output of transistor QlAl is compared with the average of the amplifier output voltage, the averaging being effected by a low-pass filter formed by resistor'Rl9 and capacitor C2. Differences between these two voltages are amplified by the differential amplifier in phase-opposition to stabilize the output voltageoperating point at ground voltage.,The input voltage to transistor Q1A1 is proportional to the current flowing through capacitor C1, and since the reactance of the capacitor- C1 is much higher than the resistance of resistor RlAI, this current will be in phase with the currents flowing in the leakage capacitance between line and ground in the sub-circuits SCI and the apparatus connected to it, and also will be proportional to that current. By adjustment of the resistor RlAl, this proportionality can be adjusted so that the output current flowing from ground-to-line is equal to the leakage current flowing from line-to-ground, to result as discussed above in a closed current loop between line L1 and ground.

It will be seen that each of the inputs Al, A2 and A3 has associated with it an input circuit 61 which, in the case of input A1 contains capacitor C1, resistor RlAl, transistor QlAl, and an emitter resistor R2Al. The other input circuits 61 are similar to that one, and each (when the associated circuit breaker is closed) will supply an ac. current through an as sociated adding resistor R3A1, R3A2 or R3A3 to the input of the differential amplifier, so that the sub-circuit 53 operates to balance the leakage current from line L1 to ground for the circuits selected by closure of the circuit breakers CB2 and CB3. Dealing now in more detail with the sub-circuit 53, the output stage is a push-pull complementary-symmetry circuit including transistors Q8, Q9, Q10 and Q11, in which the voltage between the output on the collectors of transistors Q9 and Q10 and the positive and negative supply rails (i. e., the leads at +200 volts and at 200 volts) is shared equally by transistors Q8-Q9, and Q10-Q1 l'respectively. This is effected by a voltage dividing chain of resistors R14, R15, R16 and R17, the values of which resistors are equal, and the mid-point of which resistive chain is connected to the output, i.e to line L1 through d.c. isolating capacitor C3. Emitter-follower transistor Q6 and 07 hold the emitters of transistors Q9 and Q10 at voltages mid-way between the output voltage and the positive and negative supply rail voltages, respectively. Output transistors Q8 and Q11 are drivenin phase opposition by transistors Q4 and Q5 through Zener coupling diodes CR4 and CR5 respectively. These reduce the collector-to-emitter voltage applied to transistors Q4 and 05, allowing the use of low voltage transistors in this stage.

The idling current in the output stage is determined by the voltage difference between the bases of transistors Q4 and 05, which is held at a constant value by the current flowing through forward-biassed diodes CR1, CR2 and CR3. This curphase-opposition to the input.

By adjustment of potentiometer R24A, an input voltage can be added at the base of transistor Q2 which will be in phase with the line-to-ground voltage, and which will therefore produce an output current from ground-to-line equal to the resistive leakage current flowing from line-to-ground through 1 permitted resistive leakage paths, such as through resistors Sub-circuit 55 is a duplicate of sub-circuit 53 described above, butin this case line terminal L2 is connected to input terminal B1 and so this sub-circuit monitors and compensates for leakage from line L2 to ground. It is necessary to use separate power. supplies, fed from individual windings on the power transformer, since the mid-point of the power supply as regards sub-circuit 55 is not the same signal potential as that in sub-circuit 53.

It will be appreciated that although three input terminals A1, A2, and A3 (for line L1) and three input terminals B1, B2 and B3 (for line L2) are provided, any number of such input terminals can be used, in two groups, one for each line L1 or L2, when the total number of sub-circuits to be monitored for capacitive leakage currents exceeds three.

In the use of the apparatus as illustrated in FIG. 1, the various potentiometers RlAl to RlA3, and the potentiometers R24A and R248, are adjusted in sequence to obtain a minimum reading on the meter of the ground leakage monitor. Basically, each of the two potentiometers R24A and R248 requires only one adjustment to bring the indicated fault to a minimum value, and the other potentiometers may require repeated adjustment until all leakage currents have been balanced. This adjustment can be expedited by first leaving contact breakers CB2 and CB3 open, and adjusting the potentiometers to balance only the leakage currents for the sub-circuits SCI. Next the contact breaker CB2 can be closed and adjustment made only to potentiometers RIA2 and RlB2 and possibly R24A and R248, to compensate for leakage currents in sub-circuit SC2. Finally, contact breaker CB3 can be closed andpotentiometers RlA3 and R183 adjusted to compensate for leakage currents in sub-circuit 8C3, a final adjustment of potentiometers R24A and R248 possibly being necessary.

Since the apparatus described above does not automatically adjust itself to compensate for changes in the leakage currents in the apparatus in the system, it is necessary to repeat the adjustment whenever equipment is switched into or out of circuit.

It is to be noted that when the suppressor 11 is switched off, by the use of a common operating knob for all the switches shown in it, all parts of the suppressor are isolated from the electrical supply system.

- The apparatus which has been described above enables the leakage current which can flow between a line and ground through a human body to be kept to a very small values. The apparatus is used in conjunction with a ground leakage indicator such as the indicator 13 shown, so that any major ground fault on any of the electrical circuits will be noted by that indicator. The suppressor ll acts to compensate for line-togrou'nd currents which are inherent in the use of the electrical apparatus, and which therefore cannot be dealt with by cessation of the electrical supply. It will be seen that whereas known types of ground-current protection equipment deal with fault conditions, the suppressor of the present invention deals with normal leakage currents.

By the provision of separate adjusting means for compensation of the capacitive leakage currents of various sub-circuits, it is possible to deal with the situation where the ratio of capacitive leakage current to resistive leakage current varies from sub-circuits to sub-circuit.

The above description relates to the apparatus set out in the earlier application referred to above, and is included to ensure that the present disclosure is free from insufficiency in disclosing the basic principles of the further present invention.

Referring now to FIG. 7, this Figure is generally similar to FIG. 5, and is a diagram showing how a capacitive leakage current ICl can flow through leakage capacitance CX and a resistive leakage current IRl can flow through leakage resistance RX from a line L] to ground (i.e., to metal parts of equipment) and thence through any available impedance Z to the other line L2. It will be seen that if any human body HBl bridges the insulation between line L2 and ground, it shunts the impedance Z. and the leakage current between line L1 and ground now passes partly through that human body HBl. If the impedance of the body is low compared with the impedance Z (which typically is the insulation impedance between ground and line L2), which will often be the case, the leakage current will flow for the most part through the human body.

Also shown in FIG. 7 is a current generator CGI which is arranged to drive between ground and a terminal Tl a current 17. It will be seen that, if current I7 is maintained equal to the vector sum of leakage currents lCl and [R1, no current is available on ground to flow through the human body I-IB to line L2. Putting this point more technically, on applying Kirchofi"s first law, if the conductors forming part of a network carrying a steady current meet at one point, i.e., at ground G, the sum of the currents flowing towards the point is equal to the sum of those flowing away from it. Since treating ground as said one point, IC and IR flowing towards ground are equal in vector sum to current I7 flowing away from ground, the current Il-IBl through the human body l-IBl must be zero.

Similarly, in FIG. 8, as long as the vector sum of capacitive leakage current lC2 and resistive leakage current IR2 from line 2 to ground is equal to a current 18 driven by a current generator CG2 from ground to a terminal T2, it can be shown that the current II-IB2 which can flow through a human body HB2 is zero.

It is to be noted that it is quite unimportant where current l7 or current I8 passes to in the system, although common sense excludes ground from that remark, since then Kirchoffs first law would not apply to ground treated as the said one point.

A logical development of FIGS. 7 and 8 would be FIG. 9, in which the two current generators CGI and CG2 respectively drive currents from ground to terminals T1 and T2 respectively. Proceeding further, T1 and T2 can be a common terminal T, and since it is redundant to use two current generators operating in parallel between the same two points, as shown in FIG. 10 a single current generator CG can be used acting between ground G and terminal T.

FIGs. 11, 12 and 13 show respectively three ways of connecting terminal T into the complete system, i.e., respectively to line L1, to line'L2, and to the center tap on a primary winding of transformer TR2 which has its ends connected respectively to lines L1 and L2. The secondary winding of transformer TR2 supplies the rectifier circuit furnishing dc. power to the current generator and its associated circuits. This current generator suitably is a current amplifier of theoretically infinite output impedance, and therefore is indifferent as to the voltage appearing at its output terminals, providing this voltage is within its output voltage swing capabilities.

FIGS. 14A and 14B correspond to FIGS. 6A and 6B and when taken together show a complete system for the protection ofa number of-sub-circuits. The leads which interconnect the two Figures are designated Z1 through Z7. The top part of FIG. 14A and the whole of FIG. 14B together depict a master circuit 301 while the lower part of FIG. 14A shows a number of similar networks 303 each of which protects an associated subcircuit. FIG. 14A is a simplification of the working circuit, in which the upper network 303 is included in the master circuit 301, and said further networks 303 are provided in a single unit with jack facilities for plugging in one or more blocks of additional networks.

FIGS. 14A and 14B are circuit diagrams utilizing standard symbols and comprising standard transformers, transistors, diodes, resistors and capacitors interconnected in the manner indicated. In order that the disclosure of this embodiment of the invention shall be complete, the technical details of the items shown in these Figures is listed below:

0304 MPS3702 R350 33 k. ohms 0305 MPS3702 R351 10 k. ohms 0306 MPS3711 R352 100 k. ohms 0307 MPS3711 R353 10 k. ohms 0308. MPS3711 R354 100 k. ohms 0309 MM3003 R355 1.8 megohms 0310 MM4003 R356 33 k. ohms 0311 MM4003 R357 10 k. ohms 0312' MM4003 R358 100 k. ohms 0313 MM4003- R359 10 k. ohms 0314 MM3003 R360 3.9 k. ohms 0315 MM3003 R361 4.7 k.ohms

0316 MM3003 R362 120k. ohms CAPACI'I URS R303 470 k. ohms C301 100 pf R364 27 k. ohms C302 12 pf -11 R365 set on test C303 100 pf R366 1.8 k. ohms C304 12 pf R367 1.8 k. ohms C305 0.022 f R368 '33 k. ohms C306 330 t" R369 33 k. ohms C307 470 pf R370 470 k. ohms C308. 470 pf R371 470 k. ohms C309 0001 f R373 470 k. ohms C310 0.001 1' R 374 4.7 k. ohms (.111 0.001;;1' R375 47 k. ohms (312 11001 11 R370 4.7 1;. ohms C313 Int R377 4.7 k. ohms C314 0.00mi R371! 220 ohms C315 'l tt' R379 220 ohms C316 J l R380 182 k. ohms C317 0047,11 R381 562 ohms C318 50,111 R382 1.8 megohm C319 50,111 R383 1 k. ohm C320 p 1 R384 1 k. ohm

DIODES AND RECTIFIERS LAMPS CR301 1N914 CR306 10D8 LPl NESIH (R302 1N 9l4 CR307 10D8 CR308 10D8 CR309 10D8 CR303 1N914 CR3l0 BY179 CR304 10D8 CR305 10D8 It will be seen that lines L1 and L2 of sub-circuit SC/l are connected respectively to terminals TB1-1 and TBl-2 of the uppermost of the networks 303, and that from these terminals leads 305 and 307 connect respectively to the two ends of a centertapped primary winding T? of transformer T301. The transformer has a low voltage first secondary winding TS/l arranged to energize full-wave rectifier CR310,'which provides 21 +12 volts direct voltage on lead 309 and 2-12 voltsdirect is connected through a resistor R325 to an input lead A1 of the master circuit 301, and-the slider of potentiometer R313 is connected through a resistor R327 to a second input lead A2 of the remainder of the master circuit. It will be noted that, apart from the provision of leads305 and 307, all the networks 303 are similar, each network being connected to one of the sub-circuits to be protected, and each being connecte to all three leads A1, A2 and 319. i

Considering now the remainder of the master circuit 301, the three transistors 0301, 0302 and 0303 can be considered as a group forming an operational amplifier ARI having an inverting input ,connected to lead Al and a non-inverting input (the .base of transistor 0302) connected to ground. The output of this amplifier, on the collector of transistor 0303, is fed back to itsinverting input through the resistor R352 and also is fed through resistor R354 to the inverting input of a second operational amplifier formed by transistors 0304, 0305 and 0306, the non-inverting input of which (base of transistor 0305) is connected to ground.2The output of this amplifier, on the collector of transistor 0306, is connected back to its inverting input through the resistor R358, and is connected through the capacitor C305 to the non-inverting input of a third operational amplifier AR3 formed by transistors 0307 through 0316. The inverting input (base of transistor 0308) is connected to ground through a capacitor C306.

An input signal to 0307 base, going instantaneously positive, say, produces a positive output current to the lines. This current flows to ground through the system leakage paths, enters the compensator from ground, and returns to the common of the high-voltage power supply (whence it came) through current-sensing resistor R381. g

Assuming this is conventional current flow rather than electron flow, this current makes the common point of the supply instantaneously negative with respect to ground, and this is fed hack through R380 to the base of0307 in polarity-opposition to the input. j

Thus the output amplifier 0307-0316 behaves much as an ordinary operational amplifier, except that the feed back signal is a function of output current rather than output voltage.

The output current, then, is a closely-linear function of the input current through C305 and through resistors R325, R327 etc.

R355 performs a rather useful function. Since amplifier 0304-306 does not have zero output impedance, it contributes a small component of resistive output as its capacitive output is increased. This is largely cancelled by feeding an equal amount of output from amplifier 0301-303 through R355 to 0307 base, and since 0303 output is 180 out of phase with the input, this contribution can be considered a negative-resistive component. This feature greatly speeds up the adjustment procedure.

The input lead A2 of the master unit 301 also is connected to the non-inverting input of the third operational amplifier AR3. The amplifier AR3 thususes as inputs both a signal derived from the setting of the potentiometer R301, and a signal derived from the setting of the potentiometer R313.

The output from amplifier AR3 appears on lead 321 and is applied through the capacitor C313 to the centertap ofthe primary winding of transformer T301. Thus the output current from the third operational amplifier AR3 is applied to the two lines L1 and L2 in the manner described. in connection with the earlier Figures to compensate for leakage currents from those lines to ground. t

Considering now the lower part of FIG. 14A, it will be seen that for each of the lines L1 and L2 and for each of the subcircuits SC/l through SC/3 and in fact for all the other similar sub-circuits mentioned above but not illustrated, there is a first potentiometer (e.g., potentiometer R301) permitting a presetting of a current compensating for capacitive'leakage current, and a second potentiometer (e.g., potentiometer R313) permitting a presetting of a current compensating for resistive leakage current. The phase distinction between the two currents is caused by the use of the capacitor C305 to apply the current originating on lead Al to the input of operation amplifier AR3 with substantially 90 phase shift, the current originating on lead A2 being applied to the same input directly, i.e., without a 90 phase shift. 1

1n each'network 303, the four potentiometers are adjusted to match the leakage currents to ground for their associated justed to be representative of the capacitive leakage currents,

and a second current which can be adjusted to be representa 'tive of the resistive leakage currents. It will be seen that the upper potentiometers are those involved in the balancing of capacitive leakage currents, while the lower potentiometers are those involved in balancing resistive leakage currents.

It will be seen that the output in lead 321 is coupled back to the power lines L1 and L2 through the two halves of the centertapped primary transformer winding of transformer T301.

From a comparison of the circuit shown in FIGS. 14A and 148 with the circuit shown in FIGS. 6A and 68, it will be seen that a considerableeconomy in parts has been effected by the improvements which are the subject matter of the present invention. The components needed for each added network 303 are both few and cheap, so decreasing the additional cost and increasing the reliability of the equipment.

I claim:

1. Protective means adapted to reduce the danger of electrical shock in a two-wire non-grounded electrical system, in which current generating means cause'a first current to be injected between ground and a first terminal, this current being substantially equal to and in phase with a first leakage current between a first of the two lines of the system and ground, and in which the current generating means cause a second current to be-injected between ground and a second terminal, this current being substantially equal to and in phase with a second leakage current between the second of the two lines of the said system and ground, and the said terminals being connected to atleast one of the two said lines, whereby the part of the first leakage current which can flow from ground through a body to the second line, and the part of the second leakage current which can flow from ground through a body to the first line, are substantially reduced.

2. Protective means according to claim 1, and in which the first and second terminals are directly connected together.

- 3. Protective means according to claim 1, and in which the first and second terminals are directly connected together and a single current generator serves to produce both the said first current and the said second current.

4. Protective means according to claim 2, and in which the firstand second terminals are both connected to a first only of the two said lines.

5. Protective means according to any of claims 1, and in which each of the first and second terminals is connected to a tapping on a coil winding connected at its ends respectively to the first and secondlines.

6. Protective means according to claim 2, and in which both the first and second terminals are connected to a center tap on a coil winding connected at its ends respectively to the first and second lines. 1

7. Protective means according to claim 2, and in which both the first and second terminals are connected to a center tap on a primary winding of a transformer and the two ends of the said winding are connected respectively to the said first and second lines, and a secondary winding on the transformer provides electrical energy to drive the said current generating means.

8. Protective means as claimed in any of claims 1, wherein the system is an alternating current system and the leakage currents include capacitive components.

9. Protective means as claimed in any of claims 1, wherein the system is an alternating current system, and the protective means include first adjustable means by which compensation for capacitive leakage currents is provided and second adjustable means by which compensation of resistive leakage currents is provided.

10, Protective means as claimed in any of claims 1, wherein the system includes a plurality of sub-circuits and the protective means include separate adjusting means associated with each of those sub-circuits for compensation of capacitive leakage currents in that circuit.

11. Protective means as claimed in any of claims 1, and in which the system includes a plurality of sub-circuits and the protective means includes separate adjusting means associated with each of those sub-circuits for compensation of capacitive leakage currents in that sub-circuit, the first and second termlnals are directly connected together, and a single generator serves to produce both the said first current and the said second current for all the sub-circuits.

12. A method of providing protection to reduce the danger of electrical shock in two-wire non-grounded electrical systems in which a first current is generated in phase with the leakage current from a first of the two lines of the system, this current being substantially equal to that leakage current, a second current is generated in phase with the leakage current from the second of the two lines to ground, this current being substantially equal to that second leakage current, and these two currents are applied to ground in such a manner as to neutralize the said leakage currents.

13. The method according to claim 12, in which both currents are generated in a single current generator.

14. The method according claim 13, in which the single current generator is connected between ground and one only of the two said lines.

15. The method according to claim 13, in which the single current generator is connected between ground and a center tap of a coil winding connected at its ends respectively to the two'said lines.

16. The method according to claim 15, in which the said coil winding is a primary winding of a transformer and an alternating current is applied to a primary winding of the said transformer and generates e.m.fs. in two halves of the secondary winding respectively on opposite sides of the center tap.

17. The method of claim 12, and in which the system is an alternating current system and the leakage currents include capacitive components.

18. The method of claim 12, and in which separate adjustments of the injected current are carried out to compensate respectively for capacitive and for resistive components of the leakage current.

19. A protective device suitable for connection to a twowire non-grounded electrical system and including connections respectively to ground, to a first of two lines of the system, and to a second of the two lines of the system, current generating means arranged to cause a first current to be injected between ground and a first terminal, this current being adjustable to be substantially equal to and in phase with a first leakage current between the said first line and ground, the current generating means also being arranged to cause a second current to be injected between ground and a second terminal, this current being adjustable to be substantially equal to and in phase with a second leakage current between the said second line and ground, and the said terminals being connected to at least one of the two said lines, whereby the part of the first leakage current which can flow from ground through a body to the second line, and the part of the second leakage current which can flow from ground through a body to the first line, are substantially reduced.

20. A protective device as claimed in claim 19, and in which first adjustable means enable the first current to be adjusted to match a capacitive leakage current, second adjustable means enable the first current to be adjusted to match a resistive leakage current, third adjustable means enable the second current to be adjusted to match a capacitive leakage current, and fourth adjustable means enable the second current to be adjusted to match a resistive leakage current.

21. A protective device as claimed in claim 20, and in which the system to be protected includes a plurality of sub-circuits and the protective means includes separate adjusting means associated with each of those sub-circuits for compensation for leakage currents in that sub-circuit, the said first and second terminals are connected together, and a single generator serves to produce both the said first current and the said second current for all the sub-circuits. 

1. Protective means adapted to reduce the danger of electrical shock in a two-wire non-grounded electrical system, in which current generating means cause a first current to be injected between ground and a first terminal, this current being substantially equal to and in phase with a first leakage current between a first of the two lines of the system and ground, and in which the current generating means cause a second current to be injected between ground and a second terminal, this current being substantially equal to and in phase with a second leakage current between the second of the two lines of the said system and ground, and the said terminals being connected to at least one of the two said lines, whereby the part of the first leakage current which can flow from ground through a body to the second line, and the part of the second leakage current which can flow from ground through a body to the first line, are substantially reduced.
 2. Protective means according to claim 1, and in which the first and second terminals are directly connected together.
 3. Protective means according to claim 1, and in which the first and second terminals are directly connected together and a single current generator serves to produce both the said first current and the said second current.
 4. Protective means according to claim 2, and in which the first and second terminals are both connected to a first only of the two said lines.
 5. Protective means according to any of claims 1, and in which each of the first and second terminals is connected to a tapping on a coil winding connected at its ends respectively to the first and second lines.
 6. Protective means according to claim 2, and in which both the first and second terminals are connected to a center tap on a coil winding connected at its ends respectively to the first and second lines.
 7. Protective means according to claim 2, and in which both the first and second terminals are connected to a center tap on a primary winding of a transformer and the two ends of the said winding are connected respectively to the said first and second lines, and a secondary winding on the transformer provides electrical energy to drive the said cUrrent generating means.
 8. Protective means as claimed in any of claims 1, wherein the system is an alternating current system and the leakage currents include capacitive components.
 9. Protective means as claimed in any of claims 1, wherein the system is an alternating current system, and the protective means include first adjustable means by which compensation for capacitive leakage currents is provided and second adjustable means by which compensation of resistive leakage currents is provided.
 10. Protective means as claimed in any of claims 1, wherein the system includes a plurality of sub-circuits and the protective means include separate adjusting means associated with each of those sub-circuits for compensation of capacitive leakage currents in that circuit.
 11. Protective means as claimed in any of claims 1, and in which the system includes a plurality of sub-circuits and the protective means includes separate adjusting means associated with each of those sub-circuits for compensation of capacitive leakage currents in that sub-circuit, the first and second terminals are directly connected together, and a single generator serves to produce both the said first current and the said second current for all the sub-circuits.
 12. A method of providing protection to reduce the danger of electrical shock in two-wire non-grounded electrical systems in which a first current is generated in phase with the leakage current from a first of the two lines of the system, this current being substantially equal to that leakage current, a second current is generated in phase with the leakage current from the second of the two lines to ground, this current being substantially equal to that second leakage current, and these two currents are applied to ground in such a manner as to neutralize the said leakage currents.
 13. The method according to claim 12, in which both currents are generated in a single current generator.
 14. The method according claim 13, in which the single current generator is connected between ground and one only of the two said lines.
 15. The method according to claim 13, in which the single current generator is connected between ground and a center tap of a coil winding connected at its ends respectively to the two said lines.
 16. The method according to claim 15, in which the said coil winding is a primary winding of a transformer and an alternating current is applied to a primary winding of the said transformer and generates e.m.fs. in two halves of the secondary winding respectively on opposite sides of the center tap.
 17. The method of claim 12, and in which the system is an alternating current system and the leakage currents include capacitive components.
 18. The method of claim 12, and in which separate adjustments of the injected current are carried out to compensate respectively for capacitive and for resistive components of the leakage current.
 19. A protective device suitable for connection to a two-wire non-grounded electrical system and including connections respectively to ground, to a first of two lines of the system, and to a second of the two lines of the system, current generating means arranged to cause a first current to be injected between ground and a first terminal, this current being adjustable to be substantially equal to and in phase with a first leakage current between the said first line and ground, the current generating means also being arranged to cause a second current to be injected between ground and a second terminal, this current being adjustable to be substantially equal to and in phase with a second leakage current between the said second line and ground, and the said terminals being connected to at least one of the two said lines, whereby the part of the first leakage current which can flow from ground through a body to the second line, and the part of the second leakage current which can flow from ground through a body to the first line, are substantially reduced.
 20. A protective device as claimEd in claim 19, and in which first adjustable means enable the first current to be adjusted to match a capacitive leakage current, second adjustable means enable the first current to be adjusted to match a resistive leakage current, third adjustable means enable the second current to be adjusted to match a capacitive leakage current, and fourth adjustable means enable the second current to be adjusted to match a resistive leakage current.
 21. A protective device as claimed in claim 20, and in which the system to be protected includes a plurality of sub-circuits and the protective means includes separate adjusting means associated with each of those sub-circuits for compensation for leakage currents in that sub-circuit, the said first and second terminals are connected together, and a single generator serves to produce both the said first current and the said second current for all the sub-circuits. 